The Rooppur nuclear power plant, Bangladesh’s first nuclear power plant, is under construction. The capacity of the plant is 2400 MW, with two Russian reactors (model VVER-1200 of generation III +). This is a fast-track project that is expected to be completed by 2024. Meanwhile, Bangladesh plans to build a second nuclear power plant by the sea, and it’s a prudent move. The source of cooling water is the Bay of Bengal, which has a summer water temperature of 31 degrees Celsius. Preliminary work is already underway and a few provisional sites have been selected in the southern coastal districts. Now is the time for the homework to decide on a wise choice for the second plant. The objective is to select a thermally efficient plant adapted to the climatic conditions of Bangladesh.
Before doing this, we need to assess the thermal performance of an installation and see how the temperature of the cooling water influences the efficiency of the installation. Observing modern design approaches for improving plant capacity and learning from the experiences of other nuclear power plants in Asia can help.
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Nuclear power plants up to Generation III + are essentially subcritical steam power plants. Unlike coal-fired steam power plants, they cannot reach supercritical or ultra-supercritical temperature level and achieve 40-45% higher efficiency. The reason is that the temperature of the primary coolant water in the reactor core itself has a ceiling at around 330 degrees Celsius, much lower than the critical temperature (374 ° C), and the temperature of the steam at the turbine inlet is limited to 285 degrees Celsius. . In addition, there is no possibility of superheating the steam inside the reactor.
According to the rule of thermodynamics, the higher the temperature of the steam entering the turbine and the lower the temperature in the condenser, the higher the thermal efficiency will be. Thus, advanced designs of nuclear power plants attempt to extract as much work as possible from steam, well below atmospheric pressure to increase production and efficiency. As the condenser pressure decreases, the vapor condensing temperature also decreases. The condensing temperature is chosen according to the cooling water available near the plant sites. The standard design cooling water temperature is 20 degrees Celsius, which will condense the vapor as low as 32 degrees Celsius. Thus, factories can reach the intended capacity if they are located by the sea in cold countries, but not when located in hot countries, where the sea water temperature can reach 35 degrees Celsius. Again, only a third of the thermal energy generated in the reactor core can be converted into electricity for the grid. The remaining two-thirds of the heat will be ejected into the environment. It is the cooling water that transports this unused heat to the sea or releases it into the atmosphere through cooling towers. The efficiency of nuclear power plants is thus limited to between 33 and 35%.
The temperature of the cooling water is very important for the thermal performance of steam power plants, whether they use nuclear or fossil fuel. High cooling water temperatures reduce thermal efficiency. This is why factories in cold countries are more efficient than those in hot countries. Here again, thermal performance is better in winter than in summer. For example, a plant is operating at full capacity when the cooling water temperature is 20 degrees Celsius and the steam is passed through turbines at a designed rate. When the cooling water temperature rises to 32 degrees Celsius, the turbine will experience back pressure for inadequate steam condensation. The steam flow through the turbines will decrease and the operating point will adjust to a point of higher condensing temperature and pressure. As a result, the capacity of the plant will decrease. A previous study indicates that for every degree of increase in the temperature of the cooling water, the capacity is reduced by 0.45% and the efficiency by 0.12%.
How do nuclear power plants in other regions compare? At the Barakah Nuclear Power Plant (BNPP) in Abu Dhabi in the United Arab Emirates, there are four nuclear power plants with the APR-1400 Generation 3+ reactor model from Korea. They are still under construction on the Gulf Coast, although one of them has started to operate. The total nominal capacity here is 5,600 MW.
These plants will use once-through condenser cooling water systems. The Gulf seawater’s summer temperature is 35 degrees Celsius. On the other hand, two nuclear power plants of the same model APR-1400 are in operation in Shin Kori, Korea, where the sea water temperature in summer is 20 degrees Celsius. Due to the cooling water inlet temperature difference of 15 degrees C, one unit of BNPP will produce 95 MW less than in Korea. Due to the high temperature of the cooling water, the installations will be greatly decommissioned (reduction in capacity).
BNPP also has environmental bans. The factory will shut down if the entering cooling water temperature reaches 38.5 degrees Celsius. In addition, there is a bar for the temperature of discharging water to the sea (above 40 degrees Celsius). For compliance, the plant’s cooling system will have a bypass mode to mix normal seawater with hot outlet water to lower the temperature below this bar. This restriction is likely to further reduce production.
Again, the standard condenser model designed for the 10 degree Celsius temperature range will not be suitable for the Gulf site. A larger condenser with a range of 7 degrees Celsius can be used with a higher pumping capacity for increased water flow. This will require a custom design of the cooling system.
Another power plant that can be referred to is the Kudankulam Nuclear Power Plant (KNPP) in India, built with two reactors, model VVER-1000 Generation III (made in Russia). Both plants are operational with a total nominal capacity of 2000 MW. The factories have seawater condenser cooling systems with a temperature range of 7 degrees Celsius. The summer sea water temperature is 31 degrees Celsius, and the discharge of hot water into the sea is 38 degrees Celsius, which is below the limit of 40 degrees Celsius.
According to a report, the KNPP-2 plant reached the nominal capacity of 1000 MW on January 21, 2017. It should be noted that the plant has at least reached the nominal capacity in winter, if not in summer. It is also evident that the design temperature of the condenser is close to 42 degrees Celsius. Such a model is suitable for the Indian Ocean belt with regard to thermal performance.
The examples of these plants clearly show that an energy efficient plant for Bangladesh will be one designed for tropical conditions, with a cooling water temperature of 31 degrees Celsius and a condensing steam temperature of 42 degrees Celsius. Larger capacity factories (like AP1400) are certainly attractive, but will be greatly downgraded if used in Bangladesh. This is undesirable because they add up and create a big gap between installed capacity and actual production. This difference between capacity and production is what gives the false impression of overcapacity in the power generation sector.
KM Mahbubur Rahman is a Senior Mechanical Engineer and Energy Consultant.
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